The present invention relates generally to a limit switch for a hoist or winch system to slow down and stop hoist motion at preset locations. More specifically, this invention pertains to an electronic programmable limit switch which eliminates the need for traditional mechanical components.
In a conventional hoist arrangement, a motor turns a drum through a gear system. As the drum rotates, a load is either lifted or lowered by a cable. This lifting and lowering of the load must be controlled so that the load is delivered to specific locations along the vertical range of the hoist system. In addition, the hoist system must stop the lowering or lifting at both the top and the bottom of the vertical range. Traditionally, a geared limit switch is used to stop and slow down the lift. Conventional limit switches work by using adjustable cams that trip electrical switches at preset locations. These switches are, in turn, wired to multi-function digital input terminals or in series with raise/lower run commands. Because these traditional limit switches are mechanical devices, the parts are subject to wear, tear, and vibration problems which can loosen the cams and screws within the device. This wear and tear introduces safety concerns and an increased maintenance cost for the hoist system.
Furthermore, conventional limit switches suffer from hysteresis if the direction of lifting is quickly changed around slow-down locations. Because the limit switch had been tripped after passing the slow-down location, the hoist system required an amount of time to reset the limit switch thereby causing a lag in the reaction time of the system if the direction of lifting was quickly reversed.
FIG. 1 illustrates a prior art arrangement for a hoist system. The hoist system has a motor 10 connected to a shaft 15 for turning a drum 20. A lifting cable 25 is wound about the drum 20 as the motor 10 turns the shaft 15 to lift a load. Component 30 represents a typical geared limit switch. Hoist systems are required to automatically stop the lifting or the lowering of the load when the load reaches an upper stopping location and when the load reaches a lower stopping location. Furthermore, there may be locations for slowing the load prior to reaching the top of the hoist system or the bottom of the hoist system. To accomplish this function in the typical system, the typical geared limit switch 30 has adjustable cams that trip electrical switches at preset locations. The cams and electrical switches are subject to the typical wear and tear problems of a mechanical system. As a result, the typical geared limit switch 30 introduces significant safety concerns and higher maintenance costs for the hoist system.
What is needed then is a device which eliminates the mechanical jams and hysteresis of trip switches while keeping track of the motor revolutions, thereby eliminating the safety concerns and higher maintenance costs of current hoist system arrangements.